The plants slowly choke to death, wither and dry out. They die en masse, leaves dropping and bark turning grey, creating a sea of monochrome. Since scientists first discovered Xylella fastidiosa in 2013 in Puglia, Italy, it has killed a third of the region’s 60 million olive trees – which once produced almost half of Italy’s olive oil – many of which were centuries old. Farms stopped producing, olive mills went bankrupt and tourists avoided the area. With no known cure, the bacterium has already caused damage costing about €1bn. “The greatest part of the territory was completely destroyed,” says Donato Boscia, a plant virologist and head researcher on Xylella at the Institute for Sustainable Plant Protection in Bari. A decade later, far from nearing resolution, the threat to European plants from Xylella and other diseases is only growing: in February 2024, Puglia scientists found another Xylella subspecies, which had annihilated US vineyards and never previously been detected in Italy. For many farmers, scientists and regulators, the disease is emblematic of a far broader problem: …